This application claims the priority of Japanese Patent Application No. 6-191981 filed on Jul. 22, 1994 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydrogen-absorbing alloy electrode for metal hydride alkaline batteries.
2. Description of the Prior Art
Hydrogen-absorbing alloys capable of reversibly absorbing and discharging hydrogen have been developed actively in recent years. Metal hydride alkaline batteries utilizing these hydrogen-absorbing alloys as negative electrode active material have been attracting much attention and are expected to become a mainstream in the next generation alkaline batteries, since they are lighter and can be provided with higher capacity as compared with lead, nickel-cadmium and like conventional batteries.
A process for producing hydrogen-absorbing alloy electrodes for metal hydride alkaline batteries comprises the steps of pulverizing casted blocks of a hydrogen-absorbing alloy into a powder, mixing the hydrogen-absorbing alloy powder with a binder and other ingredients to obtain a paste, coating or filling a collector with the paste and solidifying the paste by heat drying. This process is known as "paste process".
However, this process has the following problems (A) due to the presence of a binder in the obtained electrode and (B) to the hydrogen-absorbing alloy powder being produced by casting and pulverization.
(A) Problems due to the presence of a binder
1. The presence of a binder on the surface of particles of the hydrogen-absorbing alloy powder obstructs formation of 3-phase interfaces between the gas (oxygen), liquid (electrolyte) and solid (hydrogen-absorbing alloy particles), thereby retarding the reduction of the oxygen with hydrogen that converts the oxygen into water. As a result, the inside pressure of the battery increases and the electrolyte is eventually discharged together with oxygen from the battery, so that the battery has poor charge-discharge cycle characteristics. Besides, it becomes necessary to charge with a small current in order to suppress the inside pressure of the battery, so that the battery cannot be charged at a high rate. PA1 2. The presence of a binder between particles of the hydrogen-absorbing alloy lowers the electronic conductivity between the particles, thus deteriorating the high-rate discharge characteristics of the electrode. PA1 3. It becomes necessary to decrease the amount of the hydrogen-absorbing alloy powder to fill up the electrode by the volume occupied by the binder, resulting in a low packing density. PA1 1. Casting and pulverization requires a long time to cool a hydrogen-absorbing alloy melt, whereby the same elements contained therein tend to flock together and segregation phases tend to form. The metal structure thus formed non-uniformly has poor corrosion resistance. Then, the surface of the alloy is oxidized (corroded) to form an inert layer, which deteriorates the cycle characteristics. PA1 2. A separate pulverization process becomes necessary, which makes complex the manufacturing process of negative electrodes.
(B) Problems due to casting and pulverization process
To solve the problems (A) due to the presence of a binder, there has been developed a hydrogen-absorbing alloy electrode obtained by casting and pulverizing the hydrogen-absorbing alloy and then bonding the resulting powder by sintering.
However, this method cannot solve the problems (B) due to casting and pulverization process. Furthermore, it is very difficult to sinter the hydrogen-absorbing alloy particles obtained by casting and pulverization process, since the particles tend to contact with each other while forming "area contact" and hence heat diffuses upon sintering.
In order to solve the problems (B) due to casting and pulverization process, Japanese Patent Application Laid-open No. 116655/1991 proposes a hydrogen-absorbing alloy electrode obtained by the successive steps of preparing spherical or nearly spherical particles of the hydrogen-absorbing alloy by centrifugal spraying (rotary disc process) or gas atomization, adding a binder to the powder to form a paste, coating or filling a substrate (collector) with -the paste and pressing the resulting composite.
However, this hydrogen-absorbing alloy electrode cannot solve the problems (A) due to the presence of a binder.
U.S. Pat. No. 5,219,678 discloses a hydrogen-absorbing alloy electrode comprising a hydrogen-absorbing alloy powder obtained by centrifugal spraying or gas atomization and having an average particle diameter of 1 to 100 .mu.m and containing at least 70% of particles having a diameter ranging within .+-.10% of the average diameter.
However, according to a study made by the present inventors, use of a hydrogen-absorbing alloy powder having such a narrow particle size distribution, which decreases the packing density, decreases the discharge capacity and makes it difficult to obtain a metal hydride alkaline battery having excellent high-rate discharge characteristics and a long battery life.